1. Field of the Invention
The present invention relates generally to method of and apparatus for measuring horizontal frequency, and more particularly, is directed to an improvement in method of and apparatus for measuring horizontal frequency of a video signal, which is supplied to, for example, an image display equipment operative to display images based on video signals, with making use of a horizontal synchronous signal contained in the video signal.
2. Description of the Prior Art
An image display equipment provided with an image displaying screen portion on which images based on video signals are displayed, may be supplied selectively with various kinds of video signals which are different from one another in horizontal frequency, a ratio of a video signal period to a horizontal period and so on. Accordingly, there has been proposed such an image display equipment as possessing a user preset function with which a user can set the apparatus to display on its image displaying screen portion images at a size of display appropriate to the video signal supplied to the equipment.
When the user preset function accompanying the image display equipment is actually exhibited for attaining its purpose, it is necessary to measure the horizontal frequency of the video signal supplied to the image display equipment. Therefore, the image display equipment which possesses the user preset function is also provided with an apparatus for measuring the horizontal frequency of the video signal supplied thereto.
FIG. 1 shows a previously proposed apparatus for measuring the horizontal frequency of a video signal. The apparatus shown in FIG. 1 comprises an 8-bit counter 11 to which a horizontal synchronous signal SH separated from a video signal is supplied and which is operative to count horizontal synchronous pulses PH constituting the horizontal synchronous signal SH, a 16-bit counter 12 to which clock pulses CLK having a predetermined frequency are supplied to be counted, a data latch 13 for latching count date DCU obtained from the 16-bit counter 12 at a predetermined periodic timing,and a control unit 15 to which an overflow signal SOF obtained from the 8-bit counter 11, latch data DLA which are the count data DCU latched by the data latch 13 and a vertical synchronous signal SV separated from the video signal are supplied.
The control unit 15 is not provided only for constituting the apparatus for measuring the horizontal frequency of the video signal. Accordingly, an input/output device 16, an external memory device 17 and a peripheral apparatus 18, which are not related directly to the apparatus for measuring the horizontal frequency of the video signal, are connected with the control unit 15. The control unit 15 communicates with the input/output device 16, external memory device 17 and peripheral apparatus 18.
The counting operation of the 16-bit counter 12 for counting the clock pulses CLK is reset whenever the count date DCU overflows so that the 16-bit counter 12 counts continuously the clock pulses CLK as far as the clock pulses CLK are supplied thereto. When the vertical synchronous signal SV as shown in FIG. 2A is supplied to the control unit 15, the control unit 15 supplies the 8-bit counter 11 with a clear control signal CCL to reset the counting operation of the 8-bit counter 11 and supplies also the date latch 13 with a latch control signal CLA to cause the date latch 13 to latch anew the count data DCU obtained on that occasion from the 16-bit counter 12 at a time point t1 at which a predetermined waiting time T0 has passed after a time point t0 corresponding to the front edge of the vertical synchronous signal SV. The waiting time T0 is selected to correspond to a period which contains equalizing pulses and notching pulses appearing in the horizontal synchronous signal SH after the time point t0 as shown in FIG. 2B.
At the time point t1, the 8-bit counter 11 starts to count anew the horizontal synchronous pulses PH appearing in the horizontal synchronous signal SH after the time point t1 as shown in FIG. 2B to form count data DCT of the horizontal synchronous pulses PH as shown in FIG. 2C.
The data latch 13 latches the count data DCU obtained from the 16-bit counter 12 at the time point t1 and supplies the control unit 15 with the latch data DLA showing continuously a counted value Na represented by the count data DCU latched by the data latch 13 as shown in FIG. 2E. The control unit 15 to which the latch data DLA is supplied from the data latch 13 is operative to store the latch data DLA in a memory device contained therein.
After that, the 8-bit counter 11 produces the overflow signal SOF having its front edge at a time point t2 as shown in FIG. 2D to be supplied to the control unit 15 when the counted value represented by the count data DCT has exceeded xe2x80x9c255xe2x80x9d.
The control unit 15 to which the overflow signal SFO is supplied from the 8-bit counter 11 supplies again the data latch 13 with the latch control signal CLA to cause the data latch 13 to latch anew the count data DCU obtained from the 16-bit counter 12 at a time point t3 at which a lag time xcex94T resulting from a time required for data processing in the control unit 15 has passed after the time point t2. The data latch 13 latches the count data DCU obtained on that occasion from the 16-bit counter 12 at the time point t3 and supplies the control unit 15 with the latch data DLA showing continuously a counted value Nb represented by the count data DCU latched by the data latch 13 as shown in FIG. 2E. The control unit 15 to which the latch data DLA is supplied from the data latch 13 is operative to store the latch data DLA in the memory device contained therein.
After the latch data DLA showing continuously the counted value Nb are stored in the memory device contained in the control unit 15, the control unit 15 calculates the difference between the counted value Nb and the counted value Na which is shown by the latch data DLA stored previously in the memory device contained in the control unit 15, and detects a period corresponding to 256 times the horizontal period of the video signal. Then, the control unit 15 further calculates the horizontal frequency of the video signal by obtaining an inverse of {fraction (1/256)} the value representing the detected period corresponding to 256 times the horizontal period. In such a manner as described above, the horizontal frequency of the video signal is measured in the control unit 15.
As for the aforementioned image display equipment accompanied with the user preset function, it is desired for sure exhibition of the user preset function that the horizontal frequency of the video signal used in the equipment is precisely measured in such a manner that an error in measurement is restrained to be, for example, less than xc2x10.2 KHz. However, in the previously proposed apparatus for measuring the horizontal frequency of a video signal as described above, it cannot be expected to measure precisely the horizontal frequency of the video signal supplied to the apparatus.
In the case of the previously proposed apparatus mentioned above, the difference between the counted value Nb and the counted value Na calculated in the control unit 15 includes theoretically a first error of one count arising in the 16-bit counter 12 which counts the clock pulses CLK, a second error of one count at the maximum arising in the 8-bit counter 11 which results from the fact that the count data DCU obtained from the 16-bit counter 12 are latched by the data latch 13 at the time point t1 at which the waiting time T0 has passed after the time point t0 corresponding to the front edge of the vertical synchronous signal SV, and a third error which corresponds to the lag time xcex94T from the time point t2 corresponding to the front edge of the overflow signal SOF obtained from the 8-bit counter 11 to the time point t3 at which the count data DCU obtained from the 16-bit counter 12 are latched by the data latch 13.
Although the first error can be reduced so as to be substantially negligible by causing the clock pulses CLK to have a relatively high frequency, it is quite difficult to avoid the second error with which the measured horizontal frequency may include an error of 256/255xe2x88x921=0.4 percents at the maximum. Further, since the lag time xcex94T results from the time required for data processing in the control unit 15, it is theoretically possible to reduce the third error by constituting the control unit 15 with a central processing unit which performs data processing at extremely high speed and giving the calculation of the horizontal frequency an especially high priority in operational interruption in the control unit 15. However, in practice, to employ the central processing unit which performs data processing at extremely high speed causes a cost to increase in a large scale and to give the calculation of the horizontal frequency the especially high priority in operational interruption in the control unit 15 puts heavy restrictions in operation on the control unit 15 to exert a bad influence upon, for example, the peripheral apparatus 18 connected with the control unit 15. Consequently, it is actually quite difficult to reduce the third error.
Accordingly, it is an object of the present invention to provide a method of measuring horizontal frequency of a video signal, which is supplied to, for example, an image display equipment operative to display images based on video signals, with making use of a horizontal synchronous signal contained in the video signal, which avoids the aforementioned difficulties encountered with the prior art.
Another object of the present invention is to provide a method of measuring horizontal frequency of a video signal, which is supplied to, for example, an image display equipment operative to display images based on video signals, with making use of a horizontal synchronous signal contained in the video signal, which can measure precisely the horizontal frequency of the video signal without requiring data processing necessary for the measurement to be performed at extremely high speed nor with especially high priority in operational interruption.
A further object of the present invention is to provide an apparatus for measuring horizontal frequency of a video signal, which is supplied to, for example, an image display equipment operative to display images based on video signals, with making use of a horizontal synchronous signal contained in the video signal, which avoids the aforementioned difficulties encountered with the prior art.
A still further object of the present invention is to provide an apparatus for measuring horizontal frequency of a video signal, which is supplied to, for example, an image display equipment operative to display images based on video signals, with making use of a horizontal synchronous signal contained in the video signal, which can measure precisely the horizontal frequency of the video signal without requiring data processing necessary for the measurement to be performed at extremely high speed nor with especially high priority in operational interruption.
According to an aspect of the present invention, there is provided a method of measuring horizontal frequency, which comprises the steps of resetting an N-bit counter (N is a positive integer, for example, 8),which is operative to count pulses constituting a horizontal synchronous signal separated from a video signal, at a time point corresponding to a front or rear edge of a vertical synchronous signal separated from the video signal, causing a data latch portion, which is operative to latch selectively count data obtained from an M-bit counter (M is a positive integer, for example, 16) operative to count clock pulses having a predetermined frequency, to latch the count data obtained from the M-bit counter at a time point corresponding to a front or rear edge of a bit output signal obtained from a predetermined bit position of the N-bit counter, detecting a period which corresponds to X times (X is a positive integer, for example, 128) a horizontal period of the video signal based on a difference between counted values represented respectively by a couple of count data latched successively by the data latch portion, and measuring horizontal frequency of the video signal by calculating the horizontal frequency on the strength of the period corresponding to X times the horizontal period of the video signal.
According to another aspect of the present invention, there is provided an apparatus for measuring horizontal frequency, which comprises an N-bit counter operative to count pulses constituting a horizontal synchronous signal separated from a video signal and subjected to resetting at a time point corresponding to a front or rear edge of a vertical synchronous signal separated from the video signal, an M-bit counter operative to count clock pulses having a predetermined frequency, a data latch portion operative to latch selectively count data obtained from the M-bit counter at a time point corresponding to a front or rear edge of a bit output signal obtained from a predetermined bit position of the N-bit counter, and an operation control portion for detecting a period which corresponds to X times a horizontal period of the video signal based on a difference between counted values represented respectively by a couple of count data latched successively by the data latch portion and measuring horizontal frequency of the video signal by calculating the horizontal frequency on the strength of the period corresponding to X times the horizontal period of the video signal.
In each of the method of and the apparatus for measuring horizontal frequency thus constituted in accordance with the present invention, the N-bit counter, which is, for example, an 8-bit counter and operative to count the pulses constituting the horizontal synchronous signal separated from the video signal, is subjected to reset at the time point corresponding to the front or rear edge of the vertical synchronous signal separated from the video signal, and the data latch portion is caused to latch the count data obtained from the M-bit counter, which is, for example, a 16-bit counter and is operative to count the clock pulses having the predetermined frequency at the time point corresponding to the front or rear edge of the bit output signal obtained from the predetermined bit position of the N-bit counter. Then, the period, which corresponds to X times which is, for example, 128 times the horizontal period of the video signal, is detected based on the difference between the counted values represented respectively by a couple of count data latched successively by the data latch portion and the horizontal frequency of the video signal is measured by calculating the horizontal frequency on the strength of the period corresponding to X times the horizontal period of the video signal.
With the reset to which the N-bit counter is subjected at the time point corresponding to the front or rear edge of the vertical synchronous signal separated from the video signal and the latch of the count data obtained from the M-bit counter carried out in the data latch portion at the time point corresponding to the front or rear edge of the bit output signal obtained from the predetermined bit position of the N-bit counter, the difference between the counted values represented respectively by a couple of count data latched successively by the data latch portion includes theoretically only an error of one count in the M-bit counter. This error of one count in the M-bit counter can be reduced so as to be negligible in practice by using clock pulses having a relatively high frequency as the clock pulses counted by the M-bit counter. As a result, the horizontal frequency of the video signal calculated based on the difference between the counted values represented respectively by a couple of count data latched successively by the data latch portion is accurately obtained in such a manner that the error of one count in the M-bit counter is effectively reduced.
Consequently, with each of the method of and the apparatus for measuring horizontal frequency according to the present invention, the horizontal frequency of the video signal can be surely measured with high precision without requiring data processing necessary for the measurement to be performed at extremely high speed nor with especially high priority in operational interruption.
The above and other objects, features and advantages of the present invention will become apparent from the following detailed description which is to be read in conjunction with the accompanying drawings.